Nuclear energy

Nucleus of a nuclear reactor of formation and investigation TRIGA in Idaho. The Cherenkov radiation, the blue glow, can be seen.

Ikata nuclear power plant, with three pressure water reactors. Refrigeration is done through the exchange of heat with water from the ocean.

Lemoniz (Spain) nuclear power plant whose launch was abandoned by popular pressure and several attacks.

The nuclear energy or atomic energy is that which is released spontaneously or artificially in nuclear reactions. However, this term encompasses another meaning that is the use of said energy for other purposes, such as obtaining electrical, thermal and mechanical energy from atomic reactions. In this way, it is common to refer to nuclear energy not only as the result of a reaction, but as a broader concept that includes the knowledge and techniques that allow the use of this energy by the human being.

These reactions occur in the atomic nuclei of some isotopes of certain chemical elements (radioisotopes), the best known being the fission of uranium-235 (²³⁵U), with which the reactors work nuclear, and the most common in nature, inside stars, the fusion of the deuterium-tritium pair (²H-³H). However, to produce this type of energy taking advantage of nuclear reactions, many other isotopes of various chemical elements can be used, such as thorium-232, plutonium-239, strontium-90 or polonium-210 (²³²Th, ²³⁹Pu, ⁹⁰Sr, ²¹⁰Po; respectively).

There are several disciplines and/or techniques that use atomic energy as a basis and range from the generation of electrical energy in nuclear power plants to archaeological dating analysis techniques (nuclear archaeometry), nuclear medicine used in hospitals, etc.

The systems most researched and worked on to obtain usable energy from nuclear energy in a massive way are nuclear fission and nuclear fusion. Atomic energy can be transformed in an uncontrolled way, giving rise to nuclear weapons; or controlled in nuclear reactors in which electrical, mechanical or thermal energy is produced. Both the materials used and the design of the facilities are completely different in each case.

Another technique, mainly used in long-lasting batteries for systems that require little electrical consumption, is the use of radioisotope thermoelectric generators (GTR, or RTG in English), in which the different modes of disintegration are used to generate electricity in thermocouple systems from heat transferred by a radioactive source.

The energy released in these nuclear processes usually appears in the form of subatomic particles in motion. These particles, when braked in the matter that surrounds them, produce thermal energy. This thermal energy is transformed into mechanical energy using external combustion engines, such as steam turbines. Said mechanical energy can be used in transport, such as in nuclear ships.

The main characteristic of this type of energy is the high quality of energy that can be produced per unit mass of material used compared to any other type of energy known to humans, but the low efficiency of the process is surprising, since between 86% and 92% of the energy that is released is wasted.

In nuclear reactions, a huge amount of energy is usually released, partly because the mass of particles involved in this process is directly transformed into energy. This is usually explained based on the mass-energy relationship proposed by the physicist Albert Einstein.

History

Nuclear Reactions

Henri Becquerel

In 1896 Henri Becquerel discovered that some chemical elements emitted radiation. He, Marie Curie and others studied their properties, discovering that these radiations were different from the already known X-rays and had different properties, calling the three types that they managed to discover: alpha, beta and gamma.

It was soon seen that they all came from the atomic nucleus that Ernest Rutherford described in 1911.

With the discovery of the neutrino, a particle theoretically described in 1930 by Wolfgang Pauli but not detected until 1956 by Clyde Cowan and his collaborators, beta radiation could be explained.

In 1932 James Chadwick discovered the existence of the neutron that Pauli had predicted in 1930, and immediately afterwards Enrico Fermi discovered that certain radiations emitted in not very common disintegration phenomena were actually these neutrons.

During the 1930s, Enrico Fermi and his collaborators bombarded more than 60 elements, including ²³⁵uranium, with neutrons, producing the first artificial nuclear fission. In 1938, in Germany, Lise Meitner, Otto Hahn and Fritz Strassmann verified Fermi's experiments and in 1939 demonstrated that part of the products that appeared when carrying out these experiments with uranium were barium nuclei. Very soon they came to the conclusion that they were the result of the division of uranium nuclei. The discovery of fission had been made.

In France, Joliot Curie discovered that in addition to barium, secondary neutrons were emitted in this reaction, making the chain reaction feasible.

Also in 1932, Mark Oliphant theorized about the fusion of light (hydrogen) nuclei, shortly after Hans Bethe described the operation of stars, based on this mechanism.

Nuclear Fission

From left to right: J. Robert Oppenheimer, Enrico Fermi and Ernest Lawrence.

In nuclear physics, fission is a nuclear reaction, which means it takes place in the atomic nucleus. Fission occurs when a heavy nucleus splits into two or more small nuclei, plus some byproducts such as free neutrons, photons (usually gamma rays), and other fragments of the nucleus such as alpha (helium nuclei) and beta (helium electrons and positrons) particles. high energy).

During World War II, the Arms Development Department of Nazi Germany developed a nuclear power project (Uranium Project) with a view to producing a nuclear explosive device. Albert Einstein, in 1939, signed a letter to US President Franklin Delano Roosevelt, written by Leó Szilárd, warning of this fact.

On December 2, 1942, as part of the Manhattan Project directed by J. Robert Oppenheimer, the world's first man-made reactor (a natural reactor existed in Oklo) was built: the Chicago Pile-1 (CP -one).

As part of the same military program, a much larger reactor was built at Hanford for the production of plutonium, and at the same time, a cascade uranium enrichment project. On July 16, 1945, the first nuclear bomb (code name Trinity) was tested in the Alamogordo desert. This test carried out an explosion equivalent to 19,000,000 kg of TNT (19 kilotons), a power never before seen in any other explosive. Both developed projects ended with the construction of two bombs, one enriched uranium and one plutonium (Little Boy and Fat Man) that were dropped on the Japanese cities of Hiroshima (August 6, 1945) and Nagasaki (August 9, 1945).) respectively. On August 15, 1945, the Second World War in the Pacific ended with the surrender of Japan. For its part, the German nuclear weapons program (headed by Werner Heisenberg), did not reach its goal before the surrender of Germany on May 8, 1945.

Subsequently, nuclear programs were carried out in the Soviet Union (first test of a fission bomb on August 29, 1949), France and Great Britain, beginning the arms race in both blocks created after the war, reaching limits of destructive power never before suspected by humans (each side could defeat and destroy all its enemies several times).

Already in the 1940s, Admiral Hyman Rickover proposed the construction of fission reactors, this time not aimed at manufacturing material for bombs, but at generating electricity. It was rightly thought that these reactors could be a great substitute for diesel in submarines. The first test reactor was built in 1953, launching the first nuclear submarine (the USS Nautilus (SSN-571)) on January 17, 1955 at 11:00. The US Department of Defense proposed the design and construction of a usable nuclear reactor for power generation and propulsion in submarines to two different US companies: General Electric and Westinghouse. These companies developed the BWR and PWR type light water reactors respectively.

These reactors have been used for the propulsion of ships, both for military use (submarines, cruise ships, aircraft carriers,...) and civilian (icebreakers and freighters), where they have power characteristics, reduction in the size of the engines, reduction in fuel storage needs and range not surpassed by any other existing technique.

The same fission reactor designs were translated into commercial designs for electricity generation. The only changes to the design over time were increased safety measures, greater thermodynamic efficiency, increased power, and the use of new technologies as they appeared.

Between 1950 and 1960 Canada developed a new type, based on the PWR, which used heavy water as a moderator and natural uranium as fuel, instead of the enriched uranium used by light-water designs. Other reactor designs for commercial use used carbon (Magnox, AGR, RBMK or PBR among others) or molten salts (lithium or beryllium among others) as moderator. This last type of reactor was part of the design of the first nuclear-powered bomber aircraft (1954) (the US Aircraft Reactor Experiment or ARE). This design was abandoned after the development of intercontinental ballistic missiles (ICBMs).

Other countries (France, Italy, among others) developed their own designs of commercial nuclear reactors for electric power generation.

In 1946 the first fast neutron reactor (Clementine) was built at Los Alamos, using plutonium as fuel and mercury as coolant. In 1951 the EBR-I was built, the first fast reactor with which it was possible to generate electricity. In 1996, the Super Fenix, or SPX, was the most powerful fast reactor ever built (1,200 MWe). In this type of reactors, radioisotopes of plutonium, thorium and uranium that are not fissile with thermal (slow) neutrons can be used as fuel.

In the 1950s, Ernest Lawrence proposed the possibility of using nuclear reactors with geometries below criticality (subcritical reactors whose fuel could be thorium), in which the reaction would be supported by an external supply of neutrons. In 1993 Carlo Rubbia proposed using a spallation facility in which a proton accelerator would produce the necessary neutrons to maintain the facility. These types of systems are known as Accelerator Driven Systems (ADS), and the first plant of this type is expected to type (MYRRHA) begins its operation in 2033 at the center of Mol (Belgium).

Nuclear Fusion

In nuclear physics, nuclear fusion is the process by which several atomic nuclei of similar charge come together to form a heavier nucleus. Simultaneously, an enormous amount of energy is released or absorbed, which allows matter to enter a plasmatic state. The fusion of two nuclei of less mass than iron (in this element and in nickel the highest nuclear binding energy per nucleon occurs) releases energy in general. Conversely, the fusion of nuclei heavier than iron absorbs energy. In the reverse process, nuclear fission, these phenomena occur in opposite directions. Until the beginning of the 20th century, the way in which energy was generated inside the stars, necessary to counteract their gravitational collapse, was not understood. There was no chemical reaction with sufficient power and fission was not capable either. In 1938 Hans Bethe managed to explain it by means of fusion reactions, with the CNO cycle, for very heavy stars. Subsequently, the proton-proton cycle was discovered for lower mass stars, such as the Sun.

In the 1940s, as part of the Manhattan Project, the possibility of using fusion in a nuclear bomb was studied. In 1942 the possibility of using a fission reaction as an ignition method for the main fusion reaction was investigated, knowing that it could result in a power thousands of times higher. However, after the end of World War II, the development of a bomb of these characteristics was not considered essential until the explosion of the first Russian atomic bomb in 1949, RDS-1 or Joe-1. This event caused US President Harry S. Truman to announce the start of a project to develop the hydrogen bomb in 1950. On November 1, 1952, the first nuclear bomb was tested (code name Mike, part of Operation Ivy or Hiedra), with a power equivalent to 10,400,000,000 kg of TNT (10.4 megatons). On August 12, 1953, the Soviet Union carried out its first test with a thermonuclear device (its power reached a few hundred kilotons).

The conditions necessary to achieve ignition in a controlled fusion reactor, however, were not derived until 1955 by John D. Lawson. Lawson's criteria defined the minimum necessary conditions of time, density, and temperature that it had to achieve. nuclear fuel (hydrogen nuclei) for the fusion reaction to continue. However, as early as 1946 the first thermonuclear reactor design was patented. In 1951 the United States fusion program began, based on the stellarator. In the same year, the development of the first Tokamak began in the Soviet Union, giving rise to its first experiments in 1956. In 1968, this last design achieved the first quasi-stationary thermonuclear reaction ever achieved, proving that it was the most efficient design achieved to date. epoch. ITER, the international design that has a start date for its operations in 2016 and that will try to solve the existing problems to achieve a magnetic confinement fusion reactor, uses this design.

Fuel capsule prepared for the NIF inertial confining fusion reactor, filled with deuterium and tritio.

In 1962 another technique was proposed to achieve fusion based on the use of lasers to achieve an implosion in small capsules filled with nuclear fuel (again hydrogen nuclei). However, it was not until the 1970s that sufficiently powerful lasers were developed. Its practical drawbacks made this a secondary option for achieving the goal of a fusion reactor. However, due to international treaties that prohibited nuclear testing in the atmosphere, this option (basically micro-thermonuclear explosions) became an excellent test laboratory for the military, thus obtaining funding for its continuation. Thus, the American National Ignition Facility (NIF, with the start of its tests scheduled for 2010) and the French Mégajoule Laser (LMJ) have been built, which pursue the same objective of achieving a device that manages to maintain the fusion reaction from this design. None of the research projects currently underway predict a significant energy gain, so a subsequent project is planned that could lead to the first commercial fusion reactors (DEMO with magnetic confinement and HiPER with inertial confinement).

Other nuclear power systems

Volta's invention of the chemical cell in the 1800s led to a compact and portable form of power generation. From then on, the search for systems that were even smaller and that had greater capacity and duration was incessant. This type of batteries, with few variations, have been sufficient for many daily applications up to our times. However, in the 20th century, new needs arose, mainly due to space programs. Systems were then required that had a long duration for moderate electrical consumption and zero maintenance. Several solutions emerged (such as solar panels or fuel cells), but as energy needs increased and new problems appeared (solar panels are useless in the absence of sunlight), the possibility of using nuclear energy began to be studied. in these programs.

In the mid-1950s, the first investigations aimed at studying nuclear applications in space began in the United States. From these emerged the first prototypes of radioisotope thermoelectric generators (RTGs). These devices proved to be an extremely interesting alternative both in space applications and in specific terrestrial applications. These artifacts take advantage of alpha and beta decay, converting all or most of the kinetic energy of the particles emitted by the nucleus into heat. This heat is then transformed into electricity taking advantage of the Seebeck effect through thermocouples, achieving acceptable efficiencies (between 5 and 40% is usual). The radioisotopes commonly used are ²¹⁰Po, ²⁴⁴Cm, ²³⁸Pu, ²⁴¹Am, among 30 others that are they found useful. These devices achieve energy storage capacities 4 orders of magnitude higher (10,000 times higher) than conventional batteries.

In 1959, the first atomic generator was shown to the public. In 1961, the first RTG was launched into space, aboard SNAP 3. This nuclear battery, which powered a satellite of the Navy North American with a power of 2.7 W, it maintained its uninterrupted operation for 15 years.

RTG of the New Horizons (in the center below, in black), unmanned mission to Pluto. The probe was launched in January 2006 and reached its target on 14 July 2015.

These systems have been used and continue to be used in well-known space programs (Pioneer, Voyager, Galileo, Apollo and Ulysses among others). Thus, for example, in 1972 and 1973 the Pioneer 10 and 11 were launched, becoming the first of them the first human object in history to leave the solar system. Both satellites continued to function for up to 17 years after their launches.

The Ulysses mission (a joint ESA-NASA mission) was sent in 1990 to study the Sun, marking the first time that a satellite crossed both solar poles. In order to do this, the satellite had to be sent into an orbit around Jupiter. Due to the duration of the RTG that maintains its operation, the mission was extended so that another trip around the Sun could be carried out again. Although it seems strange that this satellite did not use solar panels instead of an RTG, it can be understood by comparing their weights (A 544 kg panel generated the same power as a 56 RTG). In those years there was no rocket that could send the satellite into orbit with that extra weight.

These batteries not only provide electricity, but in some cases, the heat generated is used to prevent the satellites from freezing on trips where the heat from the Sun is not enough, for example on trips outside the solar system or on missions to the poles of the Moon.

The first land-based RTG for the uninhabited island Fairway Rock lighthouse was installed in 1965, remaining in operation until 1995, at which time it was decommissioned. Many other lighthouses located in inaccessible areas near the poles (especially in the Soviet Union), used these systems. It is known that the Soviet Union manufactured more than 1,000 units for these uses.

One application given to these systems was their use as pacemakers. Until the 1970s, mercury-zinc batteries were used for these applications, which lasted about 3 years. Nuclear batteries were introduced in this decade to increase the longevity of these devices, making it possible for a young patient to have only one of these devices implanted for their entire lives. In the 1960s, the Medtronic company contacted Alcatel to design a nuclear battery, implanting the first RTG-powered pacemaker in a patient in 1970 in Paris. Several manufacturers built their own designs, but in the middle of this decade they were displaced by the new lithium batteries, which had lives of about 10 years (considered sufficient by doctors even though they had to be replaced several times until the patient died). In the mid-1980s, the use of these implants stopped, although there are still people who continue to carry this type of device.

Physical Fundamentals

Representation of the semi-disintegration period of known nuclei. The number of protons (Z) is represented in the axis of abscises, while the number of neutrons (N) is represented in the ordering axis. The isotopes marked in red are those that can be considered stable.

Sir James Chadwick discovered the neutron in 1932, the year that can be considered the beginning of modern nuclear physics.

The model of the atom proposed by Niels Bohr consists of a central nucleus composed of particles that concentrate most of the mass of the atom (neutrons and protons), surrounded by several layers of charged particles with almost no mass (electrons). While the size of the atom turns out to be of the order of the angstrom (10^-10 m), the nucleus can be measured in fermis (10^-15 m), that is, the nucleus is 100,000 times smaller than the atom.

All neutral atoms (without electric charge) possess the same number of electrons as protons. A chemical element can be identified unequivocally by the number of protons that have its core; this number is called atomic number (Z). The number of neutrons (N) however may vary for the same element. For low Z values that number tends to be very similar to protons, but by increasing Z more neutrons are needed to maintain the stability of the core. To the atoms that only distinguish the number of neutrons in their nucleus (in short, their mass), they are called isotopes of the same element. The atomic mass of an isotope is given by A=Z+N{displaystyle A=Z+N} u, the number of protons plus the number of neutrons (nucleons) that it possesses in its core.

To name an isotope, the letter indicating the chemical is usually used, with a superscript that is the atomic mass and a subscript that is the atomic number (e.g. uranium isotope 238 would be written as 92238U{displaystyle _{92}^{238}!U}).

The Core

The neutrons and protons that form the nuclei have an approximate mass of 1 or, being the proton electrically charged with positive charge +1, while the neutron has no electric charge. Taking into account only the existence of electromagnetic and gravitational forces, the nucleus would be unstable (since the same load particles would repel by undoing the nucleus), making the matter impossible. For this reason (since it is obvious that matter exists) it was necessary to add to the models a third force: the strong force (today Residual strong nuclear force). This force should have as characteristics, among others, that it was very intense, attractive at very short distances (only inside the nuclei), being repulsive at shorter distances (of the size of a nucleon), which was central in a certain range of distances, which depended on the thorn and which did not depend on the type of nucleon (neutrons or protons) on which it acted. In 1935, Hideki Yukawa gave a first solution to this new force by establishing the hypothesis of the existence of a new particle: the meson. The lightest of the mesons, the pion, is responsible for most of the potential between long-range nucleons (1 rfm). The potential of Yukawa (OPEC potential) that properly describes the interaction for two spins particles s1{displaystyle s_{1}} and s2{displaystyle s_{2}} respectively, you can write as:

V(r)=gπ π 2(mπ π c2)33(Mc2)2 2[chuckles]s1s2+S121+3Rr+3R2r2]e− − rRrR{displaystyle V(r)={frac {g_{pi }{2}{2}{2}{2}{2}{3}{3(Mc^{2}{2}{2}}{2}{2}{2}{hbar}}{2}{2}{2}{2}{2}{2}{1⁄2}}}{2}}}{2}}}{1⁄2}}}}}}}{1⁄2}}}{1⁄2}}}}}}}{1⁄2}}}{1⁄2}

Other experiments on the nuclei indicated that their form should be approximately radio spherical R=1,5⋅ ⋅ A1/3{displaystyle R=1,5cdot A^{1/3} fm, being A the atomic mass, that is, the sum of neutrons and protons. This also requires that the density of the cores be the same (Vα α R3α α A{displaystyle Valpha R^{3}alpha A}, i.e. volume is proportional to A. As density is divided by volume ρ ρ =AV=cte{displaystyle rho ={frac {A}{V}}}=cte}). This characteristic led to the equating of the nuclei with a liquid, and therefore to the model of the liquid drop, fundamental in the understanding of the nuclei fission.

Average ligature energy per nucleon of the different atomic elements according to their atomic mass.

The mass of a core, however, is not exactly the sum of its nucleons. As Albert Einstein demonstrated, the energy he maintains attached to those nucleons is the difference between the mass of the nucleus and that of its elements, and is given by the equation E=m⋅ ⋅ c2{displaystyle E=mcdot c^{2}}. So, weighing the different atoms on the one hand, and their components on the other, can be determined the average energy per nucleon that keeps attached to the different nuclei.

In the graph it can be seen how the very light nuclei have less binding energy than those that are a little heavier (the left part of the graph). This feature is the basis for the release of energy in fusion. And, conversely, on the right side it can be seen that very heavy elements have lower binding energy than those that are somewhat lighter. This is the basis of the emission of energy by fission. As can be seen, the difference on the left side (fusion) is much greater than on the right side (fission).

Fission

Typical distribution of the masses of fission products. The chart represents the case of uranium 235.

After the discovery of the neutron, Fermi carried out a series of experiments in which he bombarded different nuclei with these new particles. In these experiments he observed that when he used low energy neutrons, sometimes the neutron was absorbed, emitting photons.

To find out the behavior of this reaction, he repeated the experiment systematically on all the elements of the periodic table. So he discovered new radioactive elements, but when he got to uranium he got different results. Lise Meitner, Otto Hahn, and Fritz Strassmann managed to explain the new phenomenon by assuming that the uranium nucleus, upon capturing the neutron, split into two parts of approximately equal masses. In fact they detected barium, about half the mass of uranium. Later it was found that this cleavage (or fission) did not occur in all uranium isotopes, but only in ²³⁵U. And still later, it was learned that this split could give rise to many different elements, whose distribution of appearance is very typical (similar to the double hump of a camel).

Scheme of the phenomenon of fission ²³⁵U. A low-speed (thermal) neutron impacts a uranium core destabilizing it. This is divided into two parts and also emits an average of 2.5 neutrons per fission.

In the fission of a uranium nucleus, not only do two lighter nuclei appear as a result of the division of the uranium nucleus, but also 2 or 3 are emitted (on average 2.5 in the case of ²³⁵U) neutrons at a high speed (energy). Since uranium is a heavy nucleus, the N=Z relationship (same number of protons and neutrons) is not fulfilled, which is true for the lighter elements, so the fission products have an excess of neutrons. This excess of neutrons renders these fission products unstable (radioactive), which reach stability by disintegrating excess neutrons by beta decay, generally. The fission of ²³⁵U can occur in more than 40 different ways, thus giving rise to more than 80 different fission products, which in turn disintegrate forming decay chains, so that they finally appear near 200 elements from the fission of uranium.

The energy detached in the fission of each core ²³⁵U is on average 200 MeV. Minerals exploited for uranium extraction usually possess contents of about 1 gram of uranium per kg of ore (such as pechblenda). Like content ²³⁵U in natural uranium is 0.7 %, it is obtained that for every kilogram of mineral extracted we would have 1,8⋅ ⋅ 1019{displaystyle 1,8cdot 10^{19}} atoms of ²³⁵U. If we fixed all those atoms (1 gram of uranium) we would theoretically obtain a liberated energy of 3,6⋅ ⋅ 1027eV=5,8⋅ ⋅ 108J{displaystyle 3.6cdot 10^{27}eV=5,8cdot 10^{8}J} per gram. In comparison, by combustion of 1 kg of coal of the best quality (anthracite) you get an energy of some 4⋅ ⋅ 107J{displaystyle 4cdot 10^{7}J}, that is, more than 10 tons of anthracite (the type of coal with the greatest heat power) are needed to obtain the same energy contained in 1 kg of natural uranium.

The appearance of 2.5 neutrons for each fission makes possible the idea of carrying out a chain reaction, if it is possible to make at least one neutron of those 2.5 manage to fission a new uranium nucleus. The idea of the chain reaction is common in other chemical processes. The neutrons emitted by the fission are not immediately useful if you want to control the reaction, but you have to slow them down (moderate them) to a suitable speed. This is achieved by surrounding the atoms with another element with a small Z, such as hydrogen, carbon or lithium, a material called a moderator.

Other atoms that can fission with slow neutrons are ²³³U or ²³⁹Pu. However, fission with fast (high energy) neutrons is also possible, such as ²³⁸U (140 times more abundant than ²³⁵U) or ²³²Th (400 times more abundant than ²³⁵U).

The elementary theory of fission was provided by Bohr and Wheeler, using a model according to which the nuclei of atoms behave like liquid droplets.

Fission can also be achieved by alpha particles, protons or deuterons.

Fusion

Process of fusion between a deuterium core and a tritio. It is the most appropriate option to be carried out in a nuclear fusion reactor.

Just as fission is a phenomenon that appears naturally in the earth's crust (albeit with a small frequency), fusion is completely artificial in our environment (although the nucleus of stars is common). However, this energy has advantages over fission. On the one hand, fuel is abundant and easy to obtain, and on the other, its products are stable, light and non-radioactive elements.

In fusion, unlike fission where the nuclei divide, the reaction consists of the union of two or more light nuclei. This union gives rise to a heavier nucleus than those initially used and to neutrons. The fusion was achieved before even fully understanding the conditions that were needed in the development of weapons, limiting itself to achieving extreme conditions of pressure and temperature using a fission bomb as a starting element (Teller-Ulam Process). But it is not until Lawson defines minimum time, density and temperature criteria that fusion begins to be understood.

Although in the stars the fusion occurs between a variety of chemical elements, the element with which it is easier to reach is hydrogen. Hydrogen possesses three isotopes: common hydrogen (CNG)11H{displaystyle}{1}{1}{1}!), the deuterium (12H{displaystyle}{1}{2}{2}!) and tritium (13H{displaystyle}{1}{3}{3}!). This is because the fusion requires that the electrostatic repulsion that the nuclei experience when joining is overcome, so to a lesser electric charge, this will be less. In addition, to a greater number of neutrons, the heavier the resulting nucleus (the higher we will be in the graph of the ligature energies), with the greater the energy released in the reaction.

One particularly interesting reaction is the fusion of deuterium and tritium:

12H+13H→ → 24He+n+17,6MeV{displaystyle}{1}{2}{2}!H+{}_{1}{3}Hrightarrow {1}_{2}{4}!He+n+17,6MeV}

In this reaction, 17.6 MeV are released per fusion, more than in the other combinations with hydrogen isotopes. In addition, this reaction provides a very energetic neutron that can be used to generate additional fuel for subsequent fusion reactions, using lithium, for example. The energy released per gram with this reaction is almost a thousand times greater than that achieved in the fission of a gram of natural uranium (about seven times greater if it were a gram of pure ²³⁵U).

To overcome electrostatic repulsion, it is necessary that the cores to merge reach a kinetic energy of approximately 10 keV. This energy is obtained by intense warming (like in the stars, where temperatures of 10 are reached⁸ K), which implies a movement of atoms equal to intense. In addition to that speed to overcome electrostatic repulsion, the likelihood of the merger should be high for the reaction to happen. This implies that sufficient atoms should be possessed with sufficient energy for a minimum time. The Lawson criterion defines that the product between the density of nuclei with that energy for the time during which they must remain in that state should be n⋅ ⋅ Δ Δ =1014s⋅ ⋅ nucleors⋅ ⋅ cm− − 3{displaystyle ncdot tau =10^{14}scdot nucleoscdot cm^{-3}}.

The two methods under development to take advantage of the energy released in this reaction in a useful (non-war) way are magnetic confinement and inertial confinement (with photons coming from lasers or particles coming from accelerators).

Alpha decay

Representation of the emission of an alpha particle by a kernel.

This reaction is a form of spontaneous fission, in which a heavy nucleus emits an alpha (α) particle with a typical energy of about 5 MeV. An α particle is a helium nucleus, made up of two protons and two neutrons. In its emission the nucleus changes, so that the chemical element that undergoes this type of disintegration mutates into a different one. A typical natural reaction is as follows:

29238U→ → 29034Th+α α {displaystyle}{2}{2}_{92}{38}{hbox{U}{;to ;{}{2}{}{90}{34}{hbox{Th}};+alpha }}

In which an atom of ²³⁸U is transformed into another of ²³⁴Th.

It was in 1928 when George Gamow gave a theoretical explanation for the emission of these particles. For this, he assumed that the alpha particle lived inside the nucleus with the rest of the nucleons, in an almost independent way. Due to the tunnel effect, these particles sometimes overcome the potential well created by the nucleus, separating from it at a speed of 5% the speed of light.

Beta decay

Representation of a beta particle issued by a core.

There are two modes of beta disintegration. In type β⁻ the weak force turns a neutron (n⁰) in a proton (p⁺) and at the same time emits an electron (e⁻) and an antineutrino (.. ! ! e{displaystyle {bar {nu }}_{e}}(c):

n0→ → p++e− − +.. ! ! e{displaystyle n^{0}rightarrow p^{+}+e^{-}+{bar {nu}}}{e}}.

In type β⁺ a proton becomes a neutron emitting a positron (e⁺) and a neutrino (.. e{displaystyle nu _{e}(c):

p+→ → n0+e++.. e{displaystyle p^{+}rightarrow n^{0}+e^{+}+nu _{e}}.

However, this last mode does not occur in isolation, but requires an input of energy.

Beta decay changes the chemical element that undergoes it. For example, in β⁻ decay the element transforms into another with one more proton (and one electron). Thus in the disintegration of the ¹³⁷Cs by β⁻;

137Cs→ → 137Ba++e− − +.. ! ! e{displaystyle ^{137}Csrightarrow ^{137}Ba^{++e^{} +{bar {nu }}{e}}

In 1934, Enrico Fermi managed to create a model of this disintegration that responded correctly to his phenomenology.

Nuclear technology

Nuclear Weapons

A weapon is any instrument, means or machine that is intended to attack or defend itself. According to this definition, there are two categories of nuclear weapons:

1. Those who use nuclear energy directly for attack or defense, that is, explosives that use fission or fusion.
2. Those who use nuclear energy for propulsion, possibly using it for detonation. In this category, nuclear propulsion war vessels (cruceres, aircraft carriers, submarines, bombers, etc.) can be cited.

Atomic bomb

There are two basic ways to use nuclear energy released by explosively uncontrolled chain reactions: fission and fusion.

Fission Bomb

Methods used to create a critical mass of the fossil element used in the fission pump.

On July 16, 1945, the first man-made fission bomb explosion occurred: The Trinity Test.

There are two basic types of fission bombs: using highly enriched uranium (greater than 90% enrichment at ²³⁵U) or using plutonium. Both types are based on a runaway chain fission reaction and have only been used in actual attack on Hiroshima and Nagasaki at the end of World War II.

For this type of bomb to work, it is necessary to use an amount of the element used that is greater than the Critical Mass. Assuming 100% element richness, that's 52kg of ²³⁵U or 10kg of ²³⁹Pu. For its operation, 2 or more subcritical parts are created that are joined by means of a conventional chemical explosive in such a way that the critical mass is exceeded.

The two basic problems that had to be solved to create this type of bomb were:

• Generate sufficient quantities of the fossil element to use either enriched uranium or pure plutonium.
• To reach a design in which the material used in the bomb is not destroyed by the first explosion before reaching the criticality.

The power range of these bombs is from roughly the equivalent of one ton of TNT up to 500,000 kilotons.

Fusion Bomb

Basic Design Teller-Ullam

Following the first successful test of a fission bomb by the Soviet Union in 1949, a second generation of nuclear bombs using fusion was developed. It was called a thermonuclear bomb, H bomb or hydrogen bomb. This type of bomb has never been used against any real target. The so-called Teller-Ullam design (or secret H-bomb) separates both explosions into two phases.

These types of bombs can be thousands of times more powerful than fission bombs. In theory there is no limit to the power of these bombs, the most powerful being the Czar's bomb, with a power of more than 50 megatons.

Hydrogen bombs use a primary fission bomb that generates the pressure and temperature conditions necessary to start the fusion reaction of hydrogen nuclei. This mechanism is called the Teller-Ulam Process. The only radioactive products generated by these bombs are those produced in the primary fission explosion, which is why it has sometimes been called a clean nuclear bomb. The extreme of this characteristic are the so-called neutron bombs or N-bomb, which minimize the primary fission bomb, achieving a minimum of fission products. These bombs were also designed in such a way that the greatest amount of energy released is in the form of neutrons, with which its explosive power is one tenth that of a fission bomb. They were conceived as anti-tank weapons, since when neutrons penetrate inside them, they kill their occupants by radiation.

Nuclear-powered military ships

During World War II it was found that the submarine could be a decisive weapon, but it had a serious problem: its need to emerge after short periods to obtain air for the combustion of the diesel on which its engines were based (the invention of the snorkel improved the problem somewhat, but did not fix it). Admiral Hyman G. Rickover was the first to think that nuclear power could help with this problem.

USS Enterprise (CVN-65) along with other nuclear propulsion support vessels (a cruiser and a destroyer) in the Mediterranean. The crew forms on its deck the famous Einstein E=mc2 formula on mass-energy equivalence.

The developments of nuclear reactors allowed a new type of engine with fundamental advantages:

1. It does not require air for the operation of the engine, as it is not based on combustion.
2. A small mass of nuclear fuel allows a multi-month autonomy (even years) without refueling. For example, U.S. submarines do not need to refuel throughout their lifetime.
3. A push that no other engine can equip, so they could build submarines much bigger than those existing so far. The largest submarine built to date is the Russian Akula class (displacement of 48 000 tons, 175 m in length).

These advantages have led to ships that reach speeds of over 25 knots, can spend weeks in deep submersion, and can also store huge amounts of ammunition (nuclear or conventional) in their holds. In fact, the navies of the United States, France and the United Kingdom only have submarines that use this propulsion system.

Pressurized water, boiling water or molten salt reactors have been used in submarines. In order to reduce the weight of the fuel in these reactors, uranium with high degrees of enrichment is used (from 30 to 40% in the Russians or 96% in the Americans). These reactors have the advantage that it is not necessary (although it is possible) to convert the steam generated by heat into electricity, but can be used directly on a turbine that provides movement to the propellers that drive the ship, significantly improving the performance.

A wide variety of military ships have been built that use nuclear engines and, in some cases, carry medium or long-range missiles with nuclear warheads:

• Cruises. Like the USS Long Beach (CGN-9), two C1W-type integrated nuclear reactors.
• Destroyers. As the USS Bainbridge (CGN-25) was the smallest nuclear propulsion vessel ever built, it uses two integrated D2G nuclear reactors.
• Air carriers. The most representative is the USS Enterprise (CVN-65), built in 1961 and still operating, which uses for its propulsion 8 A2W nuclear reactors.
• Ballistic submarines. They use nuclear energy as propulsion and medium- or long-range missiles as a weapon. The Akula class is of this type, using two OK-650 nuclear reactors and carrying, in addition to conventional weaponry, 20 RSM-52 nuclear missiles, each with 10 200 kiloton nuclear heads each.
• Attack submarines. Like the Seawolf USS (SSN-21) of the Seawolf class using an integrated PWR-type S6W nuclear reactor. Speed up 30 knots.

The United States, Great Britain, Russia, China, and France all have nuclear-powered ships.

Nuclear-powered military aircraft

Both the United States and the Soviet Union considered creating a fleet of nuclear-powered bombers. In this way it was intended to keep them loaded with nuclear warheads and flying permanently near the predetermined objectives. With the development of the Intercontinental Ballistic Missile (ICBM) at the end of the 1950s, faster and cheaper, without the need for pilots and practically invulnerable, all projects were abandoned.

The experimental projects were:

• Convair X-6. American project from a B-36 bomber. He had a prototype (the NB-36H) that carried out 47 test flights from 1955 to 1957, the year in which the project was abandoned. A fission reactor was used 3 MW refrigerated with air that only came into operation for the shielding tests, never propelling the plane.
• Tupolev Tu-119. Soviet project from a Tupolev Tu-95 bomber. It didn't happen either.

Civilian nuclear propulsion

Atomic energy has been used since the 1950s as a system to push (propel) different systems, from submarines (the first to use nuclear energy), to spacecraft.

Civilian Nuclear Ships

The NS Savannah, the first ever-built nuclear vessel of goods and passengers, was launched in 1962 and dismantled 8 years later for its economic inability.

After the development of nuclear-powered ships for military use, it soon became clear that there were certain situations in which their characteristics could be transferred to civil navigation. Freighters and icebreakers have been built that use nuclear reactors for propulsion.

The first nuclear passenger-cargo ship was the NS Savannah, launched in 1962. Only three other passenger-cargo ships were built: the Japanese Mutsu, the German Otto Hahn, and the Russian Sevmorput. The Sevmorput (acronym for 'Severnii Morskoi Put'), launched in 1988 and equipped with a 135 MW KLT-40 nuclear reactor, is still in operation today transiting the Northern Sea route.

Aerospace Propulsion

Artistic recreation of the Orion Project.

Although there are several options that can use nuclear energy to power space rockets, only a few have reached advanced design levels.

The thermonuclear rocket, for example, uses hydrogen superheated in a high-temperature nuclear reactor, achieving thrusts at least twice that of chemical rockets. This type of rocket was tested for the first time in 1959 (the Kiwi 1), within the NERVA Project, canceled in 1972. In 1990 the project was relaunched under the acronym SNTP (Space Nuclear Thermal Propulsion) within the project for a manned trip to Mars in 2019. In 2003 it began under the name of Project Prometeo. Another of the possibilities contemplated for the nuclear propulsion of space rockets is the use of a nuclear reactor that feeds an ionic propellant (Nuclear Electric Xenon Ion System or NEXIS).

The Orion Project was a project devised by Stanisław Ulam in 1947, which began in 1958 at the General Atomics company. Its purpose was cheap interplanetary travel at a speed of 10% c. To do this, he used a method called pulsed nuclear propulsion (External Pulsed Plasma Propulsion is its official name in English). The project was abandoned in 1963, but the same design has been used as the basis for the British fusion-powered Project Daedalus, the American Fission Engine Project Longshot coupled with an inertial fusion engine, or Project Medusa.

The use of the GTR as a source for a radioisotope rocket has also been proposed.

Nuclear car

The only known proposal is the concept design launched by Ford in 1958: the Ford Nucleon. An operational model was never built. Its design proposed the use of a small fission reactor that could provide a range of more than 8,000 km. A prototype of the car is kept in the Henry Ford museum.

One option, included in alternatives to oil, is the use of hydrogen in fuel cells as fuel for hydrogen vehicles. Hydrogen production requires large amounts of energy. Nuclear energy could be used as an energy source, in which case the hydrogen produced could be categorized as green hydrogen since atomic energy is a low-carbon energy source.

Electricity generation

Probably the best-known practical application of nuclear power is the generation of electrical power for civilian use, particularly through the fission of enriched uranium. For this, reactors are used in which a fuel is fissioned or fused. The basic operation of this type of industrial facilities is similar to any other thermal power plant, however they have special characteristics with respect to those that use fossil fuels:

• Much stricter security and control measures are needed. In the case of fourth-generation reactors, these measures could be smaller, while the merger is expected to be unnecessary.
• The amount of fuel required annually in these facilities is several orders of magnitude lower than those required by conventional thermals.
• Direct emissions of CO2 and NOx in the generation of electricity, major greenhouse gases of antropic origin, are null and void; although indirectly, in secondary processes such as obtaining minerals and building facilities, emissions are produced.

From fission

Nuclear energy research was initially driven primarily by its military applications. However, civil applications of nuclear fission, especially in electric power generation, were also considered of great interest. Thus, December 20, 1951 was the first day that it was possible to generate electricity with a nuclear reactor (in the American EBR-I reactor, with a power of about 100 kW), but it was not until 1954 that a nuclear power plant was connected to the electricity grid (it was the Soviet Obninsk nuclear power plant, generating 5 MW with only one 17% thermal efficiency). The first commercial fission reactor was Calder Hall at Sellafield, which was connected to the electricity grid in 1956. The European Atomic Energy Community (EURATOM) was created on March 25, 1957, the same day the Community was created. European Economic Area, between Belgium, France, Germany, Italy, Luxembourg and the Netherlands. That same year the International Atomic Energy Agency (IAEA) was created. Both organizations with the mission, among others, of promoting the peaceful use of nuclear energy.

Evolution of nuclear fission plants in the world. Above: installed power (blue) and generated power (red). Below: Number of reactors built and under construction (blue and gray respectively).

Its development throughout the world experienced a great growth from that moment, in a very particular way in France and Japan, where the oil crisis of 1973 had a definitive influence, since their dependence on oil for electricity generation was very high. marked (39 and 73% respectively in those years, in 2008 they generated 78 and 30% respectively through fission reactors).^{[citation required]} In 1979 the accident of Three Mile Island caused a very considerable increase in control and security measures at the power plants, however the increase in installed capacity did not stop. But in 1986 the Chernobyl accident, in a Soviet-designed RBMK reactor that did not meet the safety requirements demanded in the West, drastically cut that growth.

In October 2007, there were 439 nuclear power plants worldwide that generated 2.7 million MWh in 2006. The installed capacity in 2007 was 370,721 MWe. As of March 2008, there were 35 plants under construction, plans to build 91 new plants (99 095 MWe) and another 228 proposals (198,995^MWe). Although only 30 countries in the world have nuclear power plants, approximately 15% of the electrical power generated in the world is produced from nuclear power. nuclear.

Most of the reactors are so-called light water reactors (LWR), which use intensely purified water as a moderator. In these reactors, the fuel used is slightly enriched uranium (between 3 and 5%).

Later it was proposed to add the fisible plutonium generated (94239Pu{displaystyle} {_{94}{239}Pu}) as extra fuel in these fission reactors, significantly increasing the efficiency of nuclear fuel and thus reducing one of the fuel problems spent. This possibility even led to the use of plutonium from nuclear weapons dismantled in the major global powers. Thus the MOX fuel was developed, which adds a percentage (between 3 and 10% in mass) of this plutonium to depleted uranium. This fuel is currently used as a percentage of conventional (enriched uranium) fuel. A mixture of torium and plutonium has also been tested in some reactors, which generates a lower amount of transuric elements.

Other reactors use heavy water as a moderator. In these reactors, natural uranium can be used, that is, without enrichment, and a fairly high amount of tritium is also produced by neutron activation. This tritium is expected to be used in future fusion plants.

Other fission projects, which have not exceeded the stage of experimentation today, are aimed at the design of reactors in which electricity can be generated from other isotopes, mainly the 90232Th{displaystyle} {_{90}{232}Th} and 92238U{displaystyle} {_{92}{238}U}.

Types of reactors

The basic difference between different nuclear fission reactor designs is the fuel they use. This influences the type of moderator and coolant used. Among all the possible combinations between type of fuel, moderator and coolant, only a few are technically viable (about 100 counting the fast neutron options). But only a few have been used so far in commercial use reactors for electricity generation (see table).

The only natural isotope that is fissionable with thermal neutrons is the 92235U{displaystyle} {_{92}{235}U}, which is in a proportion of 0.7 % in weight in natural uranium. The rest is 92238U{displaystyle} {_{92}{238}U}, considered fertile, since, although it can fission with fast neutrons, by activation with neutrons becomes 94239Pu{displaystyle} {_{94}{239}Pu}, which is fossil through thermal neutrons.

Commercial fission reactors, both first and second or third generation, use uranium with different enrichment degrees, from natural uranium to slightly enriched uranium (under 6 %). In addition, in those in which enriched uranium is used, the reactor core configuration uses different degrees of enrichment, with uranium more enriched in the center and less to the outside. This setup achieves two purposes: on the one hand reduce the leakage neutrons by reflection, and on the other hand increase the amount of 94239Pu{displaystyle} {_{94}{239}Pu} consumable. In the commercial reactors these fissible atoms are made with thermal neutrons to the maximum possible (to the degree of Burnt the fuel is called burnup), since the more profits are obtained the more the fuel is extracted.

Another isotope considered fertile with thermal neutrons is the torium (natural element, mostly composed by the isotope 90232Th{displaystyle} {_{90}{232}Th}), which by activation produces 92233U{displaystyle} {_{92}{233}U}, fossil with thermal and fast neutrons (it is general rule that those elements with atomic number A impar are fisible, and with A par fertiles).

These three isotopes are those that produce exoergic fissions, that is, they generate more energy than the one needed to produce them, with thermal neutrons. The other elements (with z-92) are fixed only with fast neutrons. So 92238U{displaystyle} {_{92}{238}U} For example, it can be fixed with power neutrons above 1.1 MeV.

A VVER-1000 reactor scale. 1- Control bars. 2- Reactor cap. 3- Chassis of the reactor. 4- Input and Output Covers. 5- Reactor vessel. 6- Active area of the reactor. 7- Fuel bars.

Although there are several ways to classify the different nuclear reactors, the most used, and with which the different types of fission reactors are called, is by the moderator/coolant combination used. These are the names of the commercial thermal neutron reactors currently used (second generation), together with their number in the world (in brackets) and their main characteristics:

• PWR (VVER in Russian). (264). Uranium enriched, moderate and cooling light water.
• BWR (94). Uranium enriched, moderate and cooling light water.
• CANDU. (43). Natural Uranium, Moderator and Coolant Heavy Water.
• AGR. (18). Uses enriched uranium as fuel, graphite moderator, CO coolant₂.
• RBMK. (12). Natural or enriched uranium, graphite moderator, refrigerant light water.
• Others. 4 Russian reactors using enriched uranium, graphite moderator and light water coolant.

Reactor designs using fast neutrons, and therefore can be used as fuel 92238U{displaystyle} {_{92}{238}U}, 94239Pu{displaystyle} {_{94}{239}Pu} or 90232Th{displaystyle} {_{90}{232}Th} among others, they do not need moderator to function. For this reason it is difficult to use the same materials used in the thermals as refrigerants, as on many occasions they also act as moderator. All the reactors of this type so far have used as refrigerant liquid metals (mercury, plutonium, potassium iodide, lead, bismuto, sodium...). When these reactors also manage to produce more fossil material than they consume, they are called fast breeding reactors. There are currently four FBRs, three in cold stop and only one in commercial operation.

Reactor designs that take advantage of the lessons learned in the intervening half century (about a dozen different designs) are called third-generation or advanced reactors. Only a few have been commissioned in Japan, and a few others are under construction. In general, they are evolutions of the second generation reactors (such as the advanced BWR or ABWR or the advanced PWR: the EPR or the AP1000), although there are some completely new designs (such as the PBMR that uses helium as coolant and TRISO fuel that contains the graphite moderator in its composition).

Fourth-generation reactors will not come off paper until at least 2020, and in general they are designs that seek, in addition to higher levels of safety than fission plants of previous generations, that the only high-activity residues have lives very short, burning the long-lived actinides. For example, accelerator-assisted jets (ADS) belong to this group. In general these reactors will be based on fast neutrons.

There are some other designs, based primarily on those described, for generating power in remote locations, such as the Russian KLT-40S floating reactor or Toshiba's 200 kW nuclear microreactor.

Security

Nuclear safety at a nuclear power plant consists of anticipating the risks associated with its activity, so that they can be analyzed and mitigated. All those systems designed to eliminate or at least minimize these risks are called protection and control systems. In a nuclear power plant for civilian use, an approach called defense in depth is used. This approach follows a multi-barrier design to achieve that purpose. A first approximation to the different barriers used (each of them multiple), from the outside in could be:

1. Regulatory Authority: is the body responsible for ensuring that the rest of the barriers are in perfect operation. It should not be linked to political or business interests, its binding decisions.
2. Rules and procedures: all proceedings must be governed by written procedures and rules. In addition, quality control must be carried out and supervised by the regulatory authority.
3. First physical barrier (responsive systems): intrinsic protection systems based on the laws of Physics that hinder the emergence of faults in the reactor system. For example the use of systems designed with negative reactivity or the use of containment buildings.
4. Second physical barrier (active systems): Reduction of the frequency with which failures can occur. It is based on redundancy, separation or diversity of security systems for the same purpose. For example the control valves that seal the circuits.
5. Third physical barrier: systems that minimize the effects due to external events at the centre itself. Like dampers that prevent a break in case of earthquake.
6. Technical barrier: all installations are installed in locations considered very safe (low probability of earthquake or vulcanism) and highly depopulated.

In addition, it must be planned what to do in the event that all or several of these levels fail due to any circumstance. Everyone, the workers or other people living nearby, must have the necessary information and training. There must be emergency plans that are fully operational. For this, it is necessary that they be periodically tested through simulations. Each nuclear power plant has two emergency plans: one internal and one external, including the external emergency plan, among other measures, evacuation plans for the nearby population in case all else fails.

Graphic with the data of the events reported to the CSN by the Spanish nuclear power plants in the period 1997-2006.

Although the safety levels of third-generation reactors have increased considerably compared to previous generations, the defense-in-depth strategy is not expected to change. For their part, the designs of future fourth-generation reactors are focusing on ensuring that all safety barriers are infallible, relying as much as possible on passive systems and minimizing active ones. In the same way, the strategy followed will probably be defense in depth.

When a part of any of these levels, made up of multiple systems and barriers, fails (due to manufacturing defects, wear, or any other reason), a warning is produced to the controllers who, in turn, are notified. communicated to the resident inspectors at the nuclear power plant. If the inspectors consider that the failure may compromise the security level in question, they notify the regulatory body (in Spain the CSN). These warnings are called reportable events. In some cases, when the failure may cause some operating parameter of the plant to exceed the Technical Operating Specifications (ETF) defined in the design of the (with certain safety margins), an automatic stoppage of the chain reaction called SCRAM is produced. In other cases, the repair of that part in question (a valve, a sprinkler, a gate,...) can be carried out without stopping the operation of the plant.

If any of the barriers fails, the probability of an accident occurring increases. If multiple barriers fail on any one level, that level may eventually be breached. If several of the levels fail, a nuclear accident may occur, which can reach different degrees of severity. These degrees of severity were organized in the International Nuclear Accident Scale (INES) by the IAEA and the AEN, starting the scale at 0 (not significant for safety) and ending at 7 (serious accident).

Throughout history there have been several nuclear accidents. The most serious was the Chernobyl accident, which occurred on November 26, 1986 at the Chernobyl nuclear power plant. In this accident, several barriers were broken: lack of governmental independence, a positive reactivity reactor design, absence of a containment building, lack of emergency plans, etc. For this reason, the accident was classified as level 7 on the International Nuclear Accident Scale (INES).

The most serious accident that occurred in Spain was the Vandellós I nuclear accident that occurred at the Vandellós I nuclear power plant on October 19, 1989. At that time, the INES scale was not used in Spain, but after its implementation, it was classified on said scale as a grade 3 incident, that is, as a "major incident", without being considered of serious accident. However, the event determined the definitive closure of the affected reactor due to the seriousness of the damage suffered.

As of the merger

Model of an ITER section.

Like fission, after its exclusively military use, the use of nuclear energy released in fusion reactions was proposed for its application in civil technology. In particular, large research projects have been directed towards the development of fusion reactors for the production of electricity. The first nuclear reactor design was patented in 1946, although it was not until 1955 that the minimum conditions that the fuel had to reach (light isotopes, usually hydrogen) were defined, called Lawson criteria, to achieve a continued fusion reaction. These conditions were first reached in a quasi-stationary manner in 1968.^{[by whom?]}

Fusion is proposed as a more efficient (in terms of energy produced per mass of fuel used), safer and cleaner option than fission, useful for the long term. However, despite numerous advances in the field, the commercial application of nuclear fusion for electric power generation is not yet available and is not expected to be for decades to come. The main difficulty encountered, among many others related to design and materials, consists in how to confine matter in the plasma state until reaching the conditions imposed by Lawson's criteria, since there are no materials capable of withstanding the imposed temperatures. Mainly, two alternatives are known to achieve Lawson's criteria, which are magnetic confinement and inertial confinement.

Currently, the ITER project, the result of international collaboration, is the most advanced in this regard, although its objective is to demonstrate the scientific feasibility of a fusion reactor, not the generation of electrical energy. The European Union is developing the design of DEMO, a nuclear reactor that will succeed ITER and that will have the objective of demonstrating the technological feasibility of fusion by serving as a prototype of a fusion nuclear power plant.

Although fusion reactions are already carried out in a controlled way in the different laboratories, at the moment the projects are in the technical feasibility study in electricity production plants such as ITER or NIF. These projects are intended to demonstrate that more energy released in a controlled nuclear reaction can be obtained than is necessary to start it, but are not intended to demonstrate the production of electrical power from this released energy. The ITER project, located in France, but which is the result of international collaboration, is based on a tokamak-type magnetic confinement reactor. Once the feasibility of achieving a fusion reactor that is capable of continuous operation for long periods has been demonstrated, prototypes will be built aimed at demonstrating its economic viability. The European Union is designing the DEMO reactor, a tokamak that will succeed ITER as a reference facility in magnetic confinement and that will have the objective of demonstrating the technological viability of energy production from nuclear fusion, serving as a first prototype of fusion nuclear power plant.

Regarding inertial confinement, the main reference is the NIF project of the Lawrence Livermore National Laboratory, in the United States. The latter achieved the objective of obtaining a positive nuclear fusion on December 5, 2022, that is, the fusion reactions triggered during the experiment released more energy than that supplied to the fuel, that is, without considering the inefficiencies of the laser and others. losses.

Types of reactors

There are two large groups, separated by the method used to achieve the conditions of time, density and temperature necessary for controlled fusion to be achieved continuously:

• Fusion by magnetic confinement.
• Fusion by inertial confinement.

In the first case, in a container where a high vacuum has been applied, the temperature of a mixture of deuterium-tritium is raised by means of electromagnetic fields until it becomes plasma.

Also, by means of electromagnetic fields, the plasma is confined in a region that is as small and as far away from the walls of the container as possible, continuously increasing the density and temperature.

This type of fusion corresponds to the designs of the Tokamak, like the future ITER, or the Stellarator, like the Spanish TJ-II.

In the second case, a very energetic and intense beam of photons or charged particles (electrons or protons) is incident on a target made up of the fuel (currently deuterium-tritium). This beam can be focused directly on the target, or indirectly on a device called a holraum built with a high-Z material that in turn generates a very intense X-ray field that is focused on the target.

Until the 1970s, lasers with the power necessary to initiate the reaction were not developed.

NIF Holraum.

Currently, it is being investigated in several centers, but at a national level. This is due to the fact that the mechanism used produces thermonuclear micro-explosions, so that both the software used in thermo-hydraulic calculations and simulations, as well as the results obtained, can be used directly in thermonuclear weapons. For this reason, the facilities built so far, in addition to seeking civil application through electricity generation, have an important military component since they allow, after the prohibition of surface nuclear tests, to carry out tests on a tiny scale (for the parameters of the weapon nuclear).

Although there are multiple designs using both lasers and particle accelerators, the most important projects so far in the world are the NIF of the United States and the French LMJ, both designs using lasers.

Security

Although the same philosophy used in fission can be used in fusion reactors, this has been raised as a non-polluting and intrinsically safe option. From the point of view of safety, since the designed reactors need an external supply of energy and fuel, if there were an accident that caused the failure of the machine, the reaction would stop, making a chain reaction impossible. uncontrolled.

The main residue from the deuterium-tritium fusion reaction would be helium, which is a noble gas and therefore does not interact with anything, including the human organism. However, nuclear fusion reactions give off highly energetic neutrons. This involves the production of radioactive materials by neutron activation. Furthermore, in a deuterium-tritium cycle, a part of the fuel itself is also radioactive (tritium). To minimize the effects, therefore:

• should be reduced as much as the amount of radioactive material used as well as that generated in the installation itself;
• the risk resulting from the handling of radioactive materials generated, whether in the form of new or recycled fuel or as radioactive waste, should be cancelled as far as possible;
• you need to define which are the best ways to manage these dumps.

To this end, research is being done on the use of low activation materials, using alloys that are not common in other applications. This aspect could reduce the amount of radioactive waste generated, but also in the event of an accident where part of the materials melted due to high temperatures, the radioactive inventory emitted would also be less. In addition, the design strategy focuses on ensuring that all the radioisotopes generated have a short half-life (less than 10 years). If this were not achieved, the strategies to be followed would be identical to those studied in the case of fission reactors.

Until the 1990s, this problem had not really arisen, so the valid materials for fusion were thought to be austenitic (SS316L and SS316-modTi) and ferritic/martensitic (HT-9 and DIN 1.1494/ MANET). Investigations had focused on waste management, leaving aside the study of possible accidents. Starting in the 1990s, it was proposed that various problems should be considered in the optimization of low-activation materials, mainly emphasizing the aspect of safety against accidents in addition to the classic aspect of waste management. From the conventional steels proposed for fusion, low activation versions were proposed, resulting from the substitution of elements that gave rise to high radioactivity by other metallurgically equivalent ones with low induced activity.

The solutions adopted in inertial or magnetic fusion in principle will not have to be the same. Thus, vanadium, titanium and chromium alloys have been developed that present better behavior in inertial fusion than in magnetic one. It is known that ceramic materials have better behavior than steels in both types of fusion.

Generation of heat and electricity from other nuclear reactions

A widely used method in those applications in which a low current electrical input is required, with a long duration, is the use of heat units through radioisotopes coupled to a series of thermocouples that provide an electrical current, the so-called radioisotope thermoelectric generators.

GTR for Voyager

In this case, the radioactivity emitted by the nuclei of some isotopes is used. The isotopes considered most interesting for this type of application are those that emit alpha particles (such as ²⁴¹Am or ²¹⁰Po), since the isotopes are more efficiently reused. emitted radiations, and its handling is easier. However, beta emitters have also been used, such as the ⁹⁰Sr.

These generators typically have lifetimes of several decades, and are extremely useful in applications where other solutions won't work. For example, in areas where it is difficult to maintain or replace the batteries and also there is not enough sunlight or wind. They have been used in lighthouses near the north pole in the former Soviet Union and are frequently used in space probes. One of its most curious applications may be its use in pacemakers.

In some space probes that must remain at a very low temperature, their ability to generate heat is simply used, ignoring the possibility of generating electricity.

On October 15, 1997, the Cassini-Huygens mission to Saturn and Titan was launched, in which one of these devices was assembled.

Security

In these devices, security is based on two main systems:

• On the one hand ensure its integrity from its continuous monitoring,
• On the other hand, get the radioactive material used to be highly inaccessible, using protections, seals or even using ceramic compositions that do not react easily with other elements.

In the case of GTR located in highly inaccessible areas, such as those used in lighthouses installed near the poles, it was assumed that the very inaccessibility of the areas ensured their integrity. This however has not prevented several accidents from happening.

In the case of those used in space satellites, the safety of radioactive materials is ensured by maintaining continuous surveillance at the facilities, both during the construction and assembly of the satellites. Once launched into space, its misuse is obviously impossible. However, GTRs have been used on some occasions in satellites in orbit around the Earth. When that orbit becomes unstable it is possible for the satellite to fall again, mostly melting on reentry. This, together with a possible launch accident, are the main security problems in this case. In total there have been 6 known accidents of this type (the last in 1996 in a Russian probe). To prevent the dispersion of the radioactive material they contain, they are made of ceramic materials (insoluble and heat resistant), surrounded by a layer of iridium, another of high-resistance graphite blocks, and a gel that provides resistance to possible re-entry into the atmosphere.

For GTRs used as pacemakers, the main problem lies in the loss of information about the patients in whom they have been used, thus making it impossible to follow them up properly. For this reason, there is the possibility that the patient, after his death, was cremated, thereby incinerating the device itself and its radioactive material.

The radioactive sources of the GTRs over which control has been lost (mainly after the fall of the USSR) are the main cause of concern for their possible use in terrorist attacks (as part of a dirty bomb), and for For this reason, great efforts are made at the international level to recover them and put them under control again.

Nuclear waste treatment

In general, any industrial application generates waste. All forms of nuclear power generation also generate them. Both nuclear fission or fusion reactors (when they come into operation) and GTRs generate conventional waste that is transferred to landfills or recycling facilities, conventional toxic waste (batteries, transformer coolant, etc.) and radioactive waste. The treatment of all of them, with the exception of radioactive waste, is identical to that given to waste of the same type generated in other places (industrial facilities, cities,...).

The treatment used for radioactive waste is different. A specific regulation was developed for them, managing them in different ways depending on the type of radioactivity they emit and the half-life they possess. This regulation encompasses all radioactive waste, whether it comes from electricity generation facilities, industrial facilities or medical centers.

Different strategies have been developed to treat the different residues that come from nuclear energy generating facilities or devices:

• Low and medium activity. In this case it is waste with short life, low radioactivity and beta or gamma radiation emitters (which may contain up to 4000 Bq g^-1 of alpha emitters of long semiperiod). They are usually materials used in the normal operations of the plants, such as gloves, rags, plastics, etc. In general they are pressed and dried (if necessary) to reduce their volume, they are hormigonated (fixed) and packed to be stored for a period of 300 or 500 years, according to countries, in controlled storages. In Spain this storage is located in the province of Córdoba (El Cabril).
• High activity. These residues have long semiperiod, high activity and contain alpha radiation emitters (if they are long semiperiod only if they exceed concentrations of activity of 4000 Bq g^-1). They are generated in much lower volume but are highly harmful immediately after being generated. Generally, although they are not the only ones, it is the very fuel bars of the already used fission reactors. Various strategies have been developed for them:

Diagram showing several high-activity waste storage systems in Yucca Mountain storage.

1. Temporary storage: in the pools of the own plants (sometimes called ATI), during the life of the center (usually 40 years), or in storages constructed on purpose. In Spain the ATC is still on the project, which will be located in the town of Villar de Cañas (Cuenca) having generated great discontent among the citizens because there is no consensus.
2. Reprocessing: in this process a physical-chemical separation of the different elements takes place, separating on the one hand those isotopes that are exploitable in other applications, civilian or military (plutonium, uranium, cobalt and cesium among others). It is the most similar option to recycling. However, in the process not all recycled elements are fully reusable, such as neptunio or americio. For these, in a volume much lower than the initial, it is still necessary to use other options such as deep geological storage.
3. Deep Geological Storage (AGP): this process consists of stabilizing fuel rods spent in containers resistant to very severe treatments that are subsequently introduced into existing mine-like locations (such as in the case of deep mines), or built for that purpose. They are usually in geological matrices known to have been stable for millions of years. The most common are limes, graníticas or salinas. The technicians estimate that these GPAs should be able to preserve the waste in full over the thousands of years in which they remain toxic without affecting people on the surface. Its main defect is that it would be very difficult or impossible to recover these wastes for useful use in the event that future techniques can efficiently use them.
4. Transmutation into new-generation nuclear power plants (Accelerator-assisted systems or rapid reactors): these systems use tortorium as additional fuel and degrade nuclear wastes in a new assisted fission cycle, which may be an alternative to the dependence of oil, although they must overcome the population's rejection. The first project will be built around 2014 (Myrrha). This technique is considered acceptable to those long-period radioisotopes for which no application has been found yet. These most problematic isotopes are the transuricas such as curio, neptunio or americio. However, the use of this technique requires additional methods, such as previous reprocessing.

To manage radioactive waste there is usually an organization created exclusively for it in each country. In Spain, the National Radioactive Waste Company was created, which manages radioactive waste of all kinds generated both in nuclear power plants and in other nuclear or radioactive facilities.

Regulation

IAEA Board of Governors

Nuclear regulation can be separated into four large groups:

1. Functions of national regulators,
2. Waste,
3. Security
4. Radiological protection.

The scientific bases of all existing international regulations are based on own studies and compilations carried out by the ICRP, UNSCEAR or the American NAS/BEIR. In addition to these, there are a number of research and development agencies in security, such as the AEN or the EPRI. From all of them, there are two international organizations that develop the bases for legislation: the IAEA (internationally) and EURATOM (in Europe).

There are also some national organizations that issue documentation dedicated to each of the fields, which serve as a guide to other countries. This is the case, for example, with the NCRP, the NRC or the US EPA, the English HPA (formerly NRPB) or the French CEA.

In addition to these specific regulations, there are other laws and agreements that are more or less related to nuclear energy. Thus, for example, the water quality laws or the OSPAR convention. Although nuclear energy is not mentioned in the Kyoto Protocol, which deals with industries that emit greenhouse gases, it does appear in other documents referring to anthropogenic global warming. Thus, in the Bonn agreements of 2001, greenhouse gas emissions trading mechanisms and technology exchange mechanisms were established, both explicitly excluding nuclear energy. In this way, the emission quotas of highly industrialized countries cannot be reduced by selling nuclear technology to less developed countries, nor can emission quotas be sold to countries that base their low emissions on nuclear energy. The IPCC, however, does recommend in its fourth report the use of nuclear energy as one of the only ways (along with renewable energy and energy efficiency) to reduce greenhouse gas emissions.

Nuclear Power Controversy

Advantages

Nuclear power plants generate approximately one third of the electrical energy produced in the European Union, thus avoiding the emission into the atmosphere of 700 million tons of carbon dioxide per year [1]^{[ citation required]} and the rest of polluting emissions associated with the use of fossil fuels.

On the other hand, the application of nuclear technology to medicine has had important contributions: radiation emissions for diagnosis, such as X-rays, and for cancer treatment such as radiotherapy; radiopharmaceuticals, which mainly consists of the introduction of substances into the body, which can be monitored from the outside. In food it has allowed, through ionizing radiation, the preservation of food. An increase in the collection of food has also been achieved, since pests have been combated, which caused crop losses.

In agriculture, we can mention radioisotopic and radiation techniques, which are used to create products with genetic modification, such as giving more color to some fruit or increasing its size.

Disadvantages

The price of the new nuclear energy is higher than that of renewable energy.

Some of these disadvantages are unlikely.

• There is a risk of contamination in case of accident or sabotage.
• It produces radioactive waste that must be stored and remain active for a long time. Although it is now easier to store it and unlike the pollution caused by global warming it can be confined.

Cost

Another drawback of atomic energy is the cost of construction and maintenance of nuclear power plants, which is very high. The latest projects that have been carried out, such as the Olkiluoto 3 plant in Finland, the Hinkley Point C plant in the United Kingdom, the Flamanville-3 plant in France and the Vogtle 3 and 4 reactors in the United States have cost between 5.3 and more than 10 million Euros per MW installed. These costs are much higher than those of renewable energy installations, since at the end of 2019 a photovoltaic installation had a cost of between €600,000 and €700,000 per MW and a wind installation around €1 million per MW. However, the amount of energy that nuclear power plants produce in their useful life compensates for their construction and maintenance costs.